It is typically necessary to run metallic lines, such as tubing carrying hydraulic fluid or fuel through aircraft structures. In many instances, such lines necessarily closely approach fuel tanks or other sources of ignitable materials. In order to prevent an electrical arc from occurring due to a lightning strike on the aircraft structure, possibly igniting these materials, it is necessary to apply a dielectric insulation to the outer surface of the metallic lines.
This is a particular problem in aircraft structures made from low-conductivity composite materials. A high current surge imposed in a composite material aircraft structure by a lightning strike will cause a high voltage to be generated between the low-conductivity structure and the metallic tubing. This generated high voltage is particularly hazardous where the tubing is supported by clamps. The dielectric insulation presently used is one or more layers of MIL-I-23053/8 (Kynar) heat shrink tubing or some form of Kapton film.
One difficulty with the use of such insulating material is that it is difficult to check the dielectric strength of the material after it has been installed on the tubing. This is particularly so after the tubing has been placed in the aircraft structure. Since the dielectric material is fragile, it can easily and unknowingly be damaged during assembly or subsequent servicing, with a resultant loss of its dielectric strength. A lightning strike could then cause a spark near a fuel tank and result in the loss of the aircraft.
Various apparatus have been proposed in the past for testing dielectric coating on tubing and the like. For example, Saw, in U.S. Pat. No. 2,920,270, discloses a high-voltage insulation tester for detecting flaws in coated pipes. Saw's invention, which is intended for use with pipes which have been buried in the ground, incorporates a test instrument having a grounding lead that can be connected to a spike driven into the ground and a high-voltage lead with a "bracelet" made from a helical metal spring. The metal spring is stretched around the outer surface of the pipe whose coating is to be tested and the voltage applied. The test instrument detects any current passing through an external leakage path which includes a break in the dielectric coating formed around the pipe. The bracelet can slide along the pipe to test the dielectric coating. However, since the spring bracelet only touches the coated pipe at those discrete points where the helix of the spring is closest to the pipe, the apparatus disclosed by Saw can provide only a rough test of the integrity of the coating on the pipe. For example, breaks in the dielectric coating which are smaller than the separation between adjacent wraps of the spring bracelet may not be detected by the apparatus disclosed in Saw, even if the bracelet makes multiple passes along the pipe.
Mantilla, in U.S. Pat. No. 2,237,187, discloses a trouble detector which checks for electrical faults by penetrating the insulation around electrical conductors. The apparatus disclosed by Mantilla therefore destroys the integrity of the dielectric coating at discrete points and, accordingly, is unsuitable for testing the integrity of the dielectric coating formed around a hydraulic fluid or fuel line. In addition, the apparatus disclosed by Mantilla is incapable of being moved along the length of the tubing while performing its testing function.
Kamper, in U.S. Pat. No. 2,229,927, discloses an electrical tester for checking automotive electrical systems. Kamper's apparatus consists of a hand-held tool having electrical connectors and indicator lights. Kamper's tool makes probing contact with electrical conductors at single points. Once a connection has been made between the tool and the electrical conductor, the tool cannot be moved along the conductor without breaking the connection.
Further, Wiseman et al., in U.S. Pat. No. 3,810,007, disclose a "holiday" detector and coating resistivity checker which incorporates a ring-shaped probe employing a sponge which is saturated with a suitable electrolyte. The probe is slipped over a pipe whose coating is to be tested and is then longitudinally moved along the pipe to test for breaks in the dielectric coating. Wiseman et al. disclose a second form of a probe having a rectangular contact surface attached to a handle for use in checking portions of nonconducting coatings on large vessels or large-diameter pipes. Wiseman et al. do not disclose a probe which can simultaneously test substantially the full circumference of a pipe which does not have at least one end accessible.
Accordingly, it is desirable to have a method and apparatus for testing a dielectric coating which can simply and efficiently test the integrity of a dielectric coating formed on the outer surface of an elongated member which does not necessarily have at least one end accessible.